Respiratory compromise is the leading cause of morbidity and mortality following cervical spinal cord injury (SCI). Accordingly, identifying treatments to enhance the ability to breathe is a high priority. We are addressing this goal by examining neuronal replacement strategies using embryonic cell transplants. Such transplants offer an opportunity to study the functional integration of "donor" neurons within the injured spinal cord. Serotonin (5-hydroxytryptamine;5-HT) plays an important role in recovery of breathing after cervical SCI. For example, respiratory recovery after SCI correlates with 5-HT immunoreactivity on phrenic motoneurons (PhrMNs) and 5-HT receptor agonists also enhance recovery. Similarly, phrenic recovery is impaired or abolished by 5-HT antagonists or ablation of serotonergic raphe neurons. Previous studies of locomotor recovery after SCI have used 5-HT cell transplants to enhance 5-HT innervation of lumbar motor pools. These 5-HT grafts can establish topographically appropriate connections and are associated with improved locomotion. Because 5-HT is critical to recovery of breathing after SCI, and also plays an essential role in initiating phrenic motor plasticity, we propose to examine the impact of cervical 5-HT cell transplant on respiratory motor recovery after cervical SCI. We hypothesize that embryonic raphe cell transplants will innervate surviving PhrMNs and improve phrenic motor output via 5-HT-dependent mechanisms. Following a cervical (C2) hemisection lesion (C2HS), ipsilateral bulbospinal pathways are disrupted leading to inactivation of PhrMNs and paralyzing the ipsilateral hemidiaphragm. Contralateral phrenic motoneurons remain active due to intact bulbospinal projections. Phrenic motoneurons ipsilateral to C2HS can be recruited via bulbospinal pathways that cross the spinal midline caudal to the injury (i.e. crossed-spinal pathways). These pathways are initially ineffective, but their synaptic efficacy gradually increases over a period of weeks to months, resulting in a partial recovery of the previously paralyzed ipsilateral hemi- diaphragm. We propose to use the C2HS model in conjunction with cervical raphe cell transplants to test our hypothesis. There have been few unequivocal demonstrations of functional recovery following cell replacement that occur via defined mechanisms. Accordingly, these studies are a proof-of-principle exploration of the potential for a phenotypically defined embryonic cell transplant (i.e. 5-HT) to enhance respiratory recovery after cervical SCI.